1. Technical Field
The present invention generally relates to the field of communications, and more particularly relates to transmitting data in a multiple-input multiple-output (MIMO) communications system.
2. Background
In the continuing evolution of wireless communications standards, such as those promulgated by the 3rd Generation Partnership Project (3GPP), increasing attention is focusing on multiple-input multiple-output (MIMO) systems. The use of multiple antennas at either or both of the transmitter and receiver in a wireless link enables several techniques for improving system capacity or reliability. Although much early work on MIMO technology emphasized space-time diversity techniques, more recent work has developed several approaches to spatial multiplexing in MIMO systems. With spatial multiplexing, data rates may be substantially increased, under certain conditions, by transmitting several data substreams in the same physical area, using the same frequencies.
There are several different techniques for obtaining these increased data rates. Some approaches require that the transmitter possess knowledge of the radio channel conditions, so-called channel state information (CSI). With perfect channel knowledge at the transmitter, the data substreams to be transmitted can be pre-processed in such a way that the received substreams are orthogonal to one another, and thus may be separately demodulated.
In practice, obtaining highly detailed and accurate CSI at the transmitter is generally impractical for at least two reasons. First, if CSI knowledge at the transmitter is based on feedback from the receiver, then the CSI is necessarily associated with an inherent delay resulting from the time lag between the measuring of the channel conditions and the use of the CSI at the transmitter. Second, sending detailed CSI from the receiver to the transmitter consumes valuable bandwidth. The more precise (and timely) the CSI, the more bandwidth is consumed. Unless the channel is varying extremely slowly, this fact requires a system design tradeoff between the accuracy of the estimates and the bandwidth consumed in feeding back the CSI.
Alternative approaches for spatial multiplexing have been developed that do not rely on precise channel knowledge at the transmitter side. Indeed, some of these techniques require no CSI at all. However, these techniques typically require that the transmitted data be coded across different substreams. This generally implies that joint decoding at the receiver end is required, resulting in high complexity. These high-complexity receivers may be impractical or too expensive for mobile devices.
One promising approach for exploiting the capacity-increasing benefits of spatial multiplexing, but with more reasonable demands on receivers, involves the use of successive interference cancellation (SIC) techniques. In an SIC receiver, data substreams are successively demodulated and decoded. Once the first data substream is demodulated, its effect on the received signal is estimated and subtracted, thus improving the receiver's ability to demodulate the succeeding streams. This approach can, in principle, be combined with any desired modulation/demodulation techniques, as well as with various coding techniques.
Several MIMO systems employing open-loop spatial multiplexing (i.e., requiring no instantaneous CSI) and SIC detection have been demonstrated. Although achieving system capacities approaching the theoretical maximums requires the use of complex multi-dimensional coding, alternative approaches where each transmit antenna radiates an independently encoded data substream have also been demonstrated. These approaches require significantly less complexity at both the transmitter and receiver ends. One example of such a system is the so-called V-BLAST architecture developed at Bell Laboratories and described in Wolniansky et al., “V-Blast: An Architecture for Realizing Very High Data Rates over the Rich-Scattering Wireless Channel”, in Proc. URSI ISSSE, September 1998, pp. 295-300, the contents of which are incorporated herein by reference. An improvement of the V-BLAST architecture, in which per-antenna rate control (PARC) and per-antenna power allocation are used to improve system capacity, is described in Chung et al., “Approaching Eigenmode BLAST Channel Capacity Using V-BLAST with Rate and Power Feedback,” in Proceedings of the Veh. Techn. Conf., October 2001, pp. 915-919, the contents of which are incorporated herein by reference. In each of these systems, SIC decoding is used to successively and separately decode data substreams transmitted from each of two or more transmit antennas. In systems employing PARC, feedback from the receiver to the transmitter may be limited to indicating the rates that should be used for the different data substreams.
In an SIC receiver, the receiver demodulates and decodes a first data substream, preferably the most reliable substream, treating the others as interference. The decoded data substream is regenerated (i.e. re-encoded and re-modulated), so that its contribution to the total received signal can be determined and subtracted from the received signal. If the substream is correctly demodulated and decoded, and if the receiver's estimate of the channel is accurate enough, then the first substream's interference to the other substreams can effectively be eliminated from the received signal. The receiver then demodulates a second substream from the modified received signal. This procedure continues until each of the data substreams of interest have been demodulated.
Because data substreams are demodulated and decoded successively, SIC detection might result in considerable delays between the decoding of the first and last data substreams. In rough terms, in the case of N data substreams, the delay associated with decoding the last data substream is N times as large as for the first data substream. (In practice, the delay is somewhat worse, as all of the data substreams except for the last must also be regenerated to remove their impact from the received signal.) The absolute delay may be reduced by augmenting the decoding resources so that each decoding operation is performed more quickly, but this may be impractical or undesirable because of the concomitant increase in hardware complexity and/or power consumption.
Another concern that arises with SIC is that of robustness. In order to successfully decode a second or subsequent data substream, each of the preceding data substreams must generally have been decoded correctly. In systems employing energy-efficient rate scheduling, i.e., most systems seeking to maximize system throughput, a receiver has little margin for error. Thus, an error in decoding an earlier-decoded data substream may result in incomplete or incorrect cancellation of that data substream from the received signal. Subtracting an erroneously decoded stream may actually worsen the interference. For this reason, if cyclic redundancy check (CRC) codes are available for each data stream, then cancellation is not attempted if the CRC does not properly compute. In any case, errors in decoding one substream will at best reduce the effectiveness of successive interference cancellation, and may often result in failed decoding of each of the subsequent data substreams.